1. Field of the Invention
The present invention relates to an image processing apparatus and an image processing method.
2. Description of the Related Art
Hitherto, researches have been extensively conducted on techniques for multiplexing, in information of an image, other information related to the image.
Recently, the so-called watermarking technology has been standardized. With the watermarking technology, additional information, e.g., the author name and note on permission of use, is multiplexed in image information, such as photographs and paintings, in a manner hard to discern visually, and the multiplexed information is circulated over a network such as the Internet.
As another application field, for the purpose of preventing fraudulent forging of paper money, stamps, stocks and bonds, etc., improved high quality of image output devices, such as printers, require a technique of burying additional information in an image to specify the output device and the body number thereof from the image printed out on paper.
For example, Japanese Patent Laid-Open No. 7-123244 proposes a technique for realizing multiplexing of information by burying additional information in a high-frequency range of a color-difference component and a saturation component in which visual sensitivity is low.
However, the above-proposed technique has problems given below.
FIG. 13 shows a general manner of burying additional information with the watermarking technology. Image information A and additional information B are multiplexed in an adder 1301 and changed to multiplexed information C. FIG. 13 shows an example in which additional information is multiplexed in a real space area of image information. If the multiplexed information C can be circulated without being subjected to image processing, e.g., various kinds of filtering, and coding, e.g., irreversible compression, it is easy to demultiplex the multiplexed information C so as to restore the additional information B with conventional techniques. Image information circulated over the Internet can be demultiplexed even after being subjected to digital filtering for improvement of image quality, such as edge emphasis and smoothing, if the circulated information has a certain level of noise durability.
It is here assumed that a multiplexed image is printed by an output device, e.g., a printer, and additional information is taken out of a resulting print. It is also assumed that the used printer provides a printer output with an expression capability of just two or several gradations per color. Recently, an ink jet printer has been commercialized which is able to express an image in several gradations per color by employing ink with a thinner dye concentration and/or by variably controlling the size of output dots. Even with such an ink jet printer, however, it is impossible to express an image in gradations comparable to those of a photograph unless the so-called pseudo gradation process is employed.
More specifically, on the above-described assumption that an image is printed out by a printer using the multiplexing method based on the watermarking technology shown in FIG. 13, the multiplexed information C is changed to quantized information D through a pseudo gradation process 1401, as shown in FIG. 14. Then, the quantized information D is printed on paper with printer output 1402, thus resulting in on-paper information (print) E that is greatly deteriorated.
In other words, for restoring the additional information from the information printed on paper to prevent the above-mentioned forging, it is required to extract the additional information B from the on-paper information E obtained after a series of processes shown in FIG. 14. Because an amount by which information is changed through both the processes 1401, 1402 is very large, it is quite difficult to multiplex additional information in a visually indiscernible manner and to properly demultiplex the multiplexed additional information from the information printed out on paper.
Furthermore, FIG. 15 shows an example of the conventional watermarking technology in which image information is converted to frequency-range information using the Fourier transform, for example, and additional information is superimposed on a high-frequency range instead of the actual space area.
Referring to FIG. 15, image information is converted to frequency-range information by an orthogonal transformation process 1501, and additional information is added by an adder 1502 in a particular frequency range that is hard to discern visually. The multiplexed information is returned again to an actual space area through an inverse orthogonal transformation process 1503. Then, as with the related art of FIG. 14, the multiplexed information is subjected to filtering that entails great changes in information, i.e., a pseudo gradation process and printer outputting.
FIG. 16 shows a process of separating additional information from on-paper information. More specifically, information of a print is input through a scanner inputting process 1601. Since the input information is an image gradated with the pseudo gradation process, it is subjected to a gradation restoring process 1602, i.e., an inverse pseudo gradation process. The gradation restoring process 1602 is generally performed using an LPF (low-pass filter). The restored information is subjected to orthogonal transformation through an orthogonal transformation process 1603, and thereafter the buried additional information is separated in a separating process 1604 based on electric power in a particular frequency range.
As seen from FIGS. 15 and 16 described above, a number of complicated processes must be passed to achieve a series of steps from multiplexing of the additional information to separation thereof. In the case of a color image, a color converting process for conversion to a color specific to a printer must be included in such a series of processing steps. To satisfactorily separate the additional information that has been subjected to those complicated processes, a signal having very high durability must be input. It is however difficult to input a highly durable signal while maintaining satisfactory image quality. Also, the necessity of a number of complicated processes means that a very long processing time is required to achieve the series of steps from multiplexing to separation of the additional information.
In the above-cited Japanese Patent Laid-Open No. 7-123244, additional information is added to a high-frequency range as described above. However, when error diffusion is performed in the pseudo gradation process in a later stage, the band of the additional information is buried in a texture band generated with the error diffusion because of characteristics of a high-pass filter specific to the error diffusion. This possibly results in a risk that demultiplexing of the additional information may be failed. Also, a scanner with a very high accuracy is required for the demultiplexing of the additional information.
Thus, assuming the pseudo gradation process to be executed, the conventional methods shown in FIGS. 14 and 15 are not suitable. In other words, a method of multiplexing additional information is required which sufficiently takes into consideration characteristics of the pseudo gradation process.
Japanese Patent Nos. 2640939 and 2777800 propose examples in which multiplexing of additional information and redundancy of the pseudo gradation process are combined with each other.
With the former Japanese Patent No. 2640939, in a process of binary-coding using a systematic dithering method, data is mixed in an image signal by selecting one of dither matrices that represent the same gradation.
In the case using the systematic dithering method, however, it is difficult to print out an image with high quality comparable to a photographic unless a printer having high resolution and very excellent mechanical accuracy is employed. The reason is in that a slight deviation in mechanical accuracy causes low-frequency noises in the form of streaks, for example, and those noises are visually perceived on paper with ease.
Also, when the dither matrix is cyclically changed, the band of particular frequency generated by a regularly arrayed dither is disturbed and image quality is adversely affected.
Further, the gradation expressing capability is greatly different depending on the type of dither matrix. On paper, particularly, a change of area rate due to, e.g., overlaps between dots, differs depending on the dither matrix. Even in an area where density is uniform in terms of signal, therefore, a density change may occur upon switching of the dither matrix.
Moreover, a possibility of false demultiplexing is very high when the demultiplexing (separating) side employs a demultiplexing method that must estimate which dither matrix was used for binary-coding, in a state where a pixel value of image information as an original signal is uncertain.
The above-cited latter Japanese Patent No. 2777800 proposes a method of multiplexing additional information with color dither patterns based on arrays of those patterns. In this proposed method, deterioration of image quality is unavoidable upon switching of the dither pattern as with the above-described former proposal. Further, comparing with the former proposal, a larger quantity of additional information can be multiplexed, but color changes are caused with changes in array of color components and deterioration of image quality is increased particularly in a flat portion. Additionally, it is thought that demultiplexing of on-paper information is more difficult to achieve.
Anyway, those proposed methods of changing the dither matrix has the problem in that deterioration of image quality is serious, while demultiplexing is comparatively difficult.
In view of the above-stated situations in the art, the applicant of this application has proposed, in Japanese Patent Laid-Open No. 2000-287062, a method of burying codes by artificially creating a combination of quantized values, which are not generated with the usual pseudo gradation process, with the aid of a texture generated with the error diffusion process.
With that proposed method, since the texture shape is just a little changed microscopically, image quality is not deteriorated from the point of visual sense. Also, by employing a method of changing a quantizing threshold in the error diffusion process, a density value of area gradation is also maintained from the point of visual sense. Therefore, multiplexing of different types of signal can be very easily realized.
In the above-proposed method, however, whether the texture is an artificial one or not must be determined on the demultiplexing side. In the case of a print obtained by printing out information on paper, the texture cannot be sometimes satisfactorily reproduced because of deviations from the desired impact positions of ink droplets, e.g., dot twists.
For a color image, there is mainly practiced a method of multiplexing additional information in a color component that has the lowest sensitivity from the point of visual sense. However, determining a texture in an actual space area is easily affected by other color components, and a difficulty arises in separating the multiplexed information.
Further, to overcome the above-mentioned problem, the applicant of this application has proposed, in Japanese Patent Laid-Open No. 2001-148778, etc., a method of modulating the amplitude of a quantizing threshold itself in the error diffusion process at predetermined periodicity, controlling the periodicity of the threshold modulation in plural kinds for each unit area, thereby controlling the probability of occurrence of quantized values in the pseudo gradation process, and burying codes based on that periodicity.
Comparing with the aforementioned method of determining the texture position and shape, such a proposed method can realize more satisfactory demultiplexing of information printed out on paper because an important factor for the multiplexing is provided by information of relative electrical power between a plurality of predetermined frequency bands instead of phase information that forms codes.
In that proposed method, however, processing conditions for each color component in the pseudo gradation process are not disclosed regarding specific correlation between colors. Thus, there is still a demand for further improvements in a method of reducing, from the point of visual sense, noises caused with the process of burying additional information in printed-out information on paper, and a method of reducing an extraction error rate when extracting the additional information.